Bandpass sigma-delta modulator

ABSTRACT

A bandpass sigma-delta modulator using acoustic resonators or micro-mechanical resonators. In order to improve resolution at high frequencies, acoustic resonators or micro-mechanical resonators are utilized in a sigma-delta modulator instead of electronic resonators. The quantized output is fed back using a pair of D/A converters to an input summation device. In fourth order devices, the feed back is to two summation devices in series. Such a sigma-delta modulator is usable in a software defined radio cellular telephone system and in other applications where high-frequency and high-resolution A/D conversion is required.

This application claims priority on provisional Application No.60/331,256 filed Nov. 13, 2001, the entire contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to a bandpass sigma-deltamodulator, and more particularly to a bandpass sigma-delta modulatorusing an acoustic resonator or micro-mechanical resonator.

DESCRIPTION OF THE BACKGROUND ART

Cellular telephone systems have become very popular in many countriesthroughout the world. Unfortunately, the specific standards adopted bydifferent countries are often different and cellular devices from onecountry will not be operable within another system. Accordingly,completely different handsets are necessary if a person is operating intwo different countries having different standards.

As a result of this difficulty, there have been some efforts to providea single device that is operable in different countries having differentstandards. A technology known as software-defined radio (SDR) providesone solution to this problem. In the SDR system, the entire band of RFor IF signal is digitized and the channel is selected using aprogrammable digital filter. Thus, the SDR can be reconfigured throughthe software to suit different standards. However, this effort has notbeen completely successful because it requires a high-speed A/Dconverter that must have not only high-speed, but also provide adequateresolution. In the ideal SDR arrangement, the RF is directly digitizedin the receiver. It requires that the A/D converter have a speed in thegigahertz range, and also have a dynamic range over a 100 dB, which isequivalent to a 16-bit resolution. Among the various A/D converters, thesigma-delta A/D converter has promise in achieving the desired dynamicrange. However, it is only achievable at a much lower frequency band. Asthe frequency increases, circuit imperfections become dominant anddegrade the dynamic range of the A/D converter. At the gigahertzsampling frequency range, the highest dynamic range for the reportedbandpass sigma-delta modulator is 75 dB, which only corresponds to a12.5 bit resolution.

A sigma-delta A/D converter consists of a sigma-delta modulator and adigital filter. FIG. 1 shows a typical arrangement of a bandpasssigma-delta modulator. The key element in the modulator is the resonator2 which provides the quantization noise shaping. That is, the resonatoracts as a bandpass filter in a band around its resonance frequency. Theresonator needs to operate at a high frequency and have a high Q(quality factor) value. The output of the resonator is passed to aquantizer 3 which produces a digital “1” signal if its input exceeds athreshold and a digital “0” signal if the input is less than thethreshold. This digital signal of a series of 0's and 1's becomes theoutput of the sigma-delta modulator. These signals are also fed back toa D/A converter 4, and the resultant analog signal is applied as asecond input to summation device 1 which also receives the input to themodulator. The difference between the input and the feedback of thesummation device produces an input to the resonator.

FIG. 2 shows a typical output spectrum from a fourth order bandpasssigma-delta modulator where the quantization noise is shaped away fromthe resonance frequency, resulting in a very high dynamic range. Thedepth of the notch is related to the Q value of the resonator. Thehigher the Q value is, the deeper the notch. The noise shaping is alsodependent on the order of the modulator. Higher order modulators providebetter noise shaping and hence a higher dynamic range.

Typically, the resonator is made of one of three different electroniccircuits, namely, a passive L-C tank, a transconductor-capacitor or aswitch-capacitor. However, none of these circuits have been successfulin the situation described. The first two circuits cannot achieve a highQ value due to parasitic losses and non-linearity. Typical Q values arearound 10 and 40 for the integrated L-C tank with and without Qenhancement, respectively. The enhanced Q value for thetransconductor-capacitor resonator can be up to 300 at a frequency ofseveral hundred MHz. The switch-capacitor resonator is restricted by itslow resonant frequency (<100 MHz) due to the slow settling behavior ofthe circuit. The use of these types of resonators prevents thesigma-delta modulator from achieving a high speed and high dynamic rangeat the same time as is required in an SDR situation. In order to achievea workable SDR system, it is necessary to find a resonator which isusable in a sigma-delta modulator to achieve high speed and high dynamicrange.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide a bandpasssigma-delta modulator having high-speed and high-dynamic range.

Another object of the invention is to provide a bandpass sigma-deltamodulator using a micro-mechanical resonator.

Another object of the invention is to provide a bandpass sigma-deltamodulator utilizing an acoustic resonator.

A further object of this invention is to provide a second order bandpasssigma-delta modulator having two D/A converters.

A still further object of this invention is to provide a fourth ordersigma-delta modulator having two D/A converters and two summationdevices.

A still further object of this invention is to provide a sigma-delta A/Dconverter including a sigma-delta modulator having a resonator of themicro-mechanical or acoustical type.

Briefly, these and other objects of the invention are achieved by usingeither a micro-mechanical resonator or an acoustical resonator as abandpass filter which provides an output to a quantizer. The output ofthe quantizer acts as the output of the modulator and is also fed backto two different D/A converters. The output of the two converters isadjusted by a gain and applied to a summation device, along with theinput. The output of the summation device is applied as an input to theresonator. In the fourth order device, two resonators are used alongwith a second summation device.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a prior art bandpass sigma-delta modulator.

FIG. 2 is a graph of the output spectrum from a typical fourth ordermodulator.

FIG. 3 is a block diagram of a second order bandpass sigma-deltamodulator using a micro-mechanical resonator according to the presentinvention.

FIG. 4 is a graph of the output spectrum from the second order bandpasssigma-delta modulator of FIG. 3.

FIG. 5 block diagram of a fourth order bandpass sigma-delta modulatorusing micro-mechanical resonators.

FIG. 6 is a graph of the output spectrum from a fourth order bandpasssigma-delta modulator according to FIG. 5.

FIG. 7 is a block diagram of the second order bandpass sigma-deltamodulator using an acoustical resonator.

FIG. 8 is a graph of the output spectrum from a second order bandpasssigma-delta modulator shown in FIG. 7.

FIG. 9 is a block diagram of a fourth order bandpass sigma-deltamodulator using acoustical resonators.

FIG. 10 is a graph showing the output spectrum from a fourth orderbandpass sigma-delta modulator as shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical corresponding parts throughout the several views, and moreparticularly to FIG. 3 which shows a second order sigma-delta modulatorusing a micro-mechanical resonator. The summing circuit 1 adds theanalog input to two outputs from two gain stages. The sum forming theoutput of the summing circuit becomes the input to the micro-mechanicalresonator 2. The resonator selectively passes the signal at itsresonance frequency and attenuates signals at other frequencies. Theoutput from the resonator is adjusted by gain stage G1, 6 and fed toquantizer 3. The signal is quantized to form a series of binary bits asin FIG. 1 which form the output of the modulator. The output is also fedback as an input to a pair of D/A converters, DAC1, 4, and DAC2, 5. Theconverters are both one bit converters but have different output pulses.One is a return to zero converter, whereas the other is a half return tozero converter. As a result, they have different transfer functions inthe frequency domain. The output of the converters are analog signalswhich are fed back to the summing circuit through gain stages G2, 7 andG3, 8. The value of the gains in G1-G3 may be either positive ornegative and often the gains in the feedback path are negative so thatthe feedback is subtracted from the input signal in a similar fashion toFIG. 1.

Since the micro-mechanical resonator 2 is a second order system, thebandpass modulator shown in FIG. 3 is of the second order, and providessecond order noise-shaping as shown in FIG. 4. This modulator based onthe micro-mechanical resonator is capable of digitizing the analog inputsignal, but with a limited resolution. Better resolution can be obtainedusing a fourth order modulator as will be described below. However,since the Q values of the micro-mechanical resonator can easily begreater than 1000 and even as high as 10,000, it is possible to have ahigh resonance frequency, up to a gigahertz using this type ofresonator. Accordingly, the use of this different resonator allows themodulator to operate at the gigahertz sampling frequency range with goodresolution.

FIG. 5 illustrates a corresponding fourth order bandpass sigma-deltamodulator using a micro-mechanical resonator. Input summing circuit 1adds the input to the modulator and the feedback from 2 gain stages in asimilar fashion to FIG. 3. The output of the summing circuit is fed tomicro-mechanical resonator 2 which produces an output to gain stage G1,6. This output is then applied to the second summing circuit 9 alongwith two additional feedback signals. The sum of these signals is usedas the input to the second micro-mechanical resonator 10 which alsoselectively passes the signal at the resonance frequency and attenuatesthe signal at other frequencies. This output is adjusted by gain stageG4, 11 and its output is applied to quantizer 3. The output of thequantizer is a series of digital bits as described above and forms theoutput of the modulator. This output is fed back to two converters D/Aconverters 4 and 5 in a similar fashion to FIG. 3. However, in additionto the analog feedback signal being applied to gain stages G2 and G3 asin FIG. 3, they are also applied to gain stages G5, 12, and G6, 13 asinputs to the second summing circuit 9.

The output is the output of a fourth order bandpass sigma-deltamodulator having a digitized form of the input analog signal. By havingtwo micro-mechanical resonators in the loop, this device provides fourthorder noise-shaping as shown in FIG. 6. This fourth order modulatorusing two micro-mechanical resonators is able to provide digitization ofthe analog signal with high resolution.

As in the embodiment of FIG. 3, the gain stages can be either positiveor negative, and either gain or attenuation, as necessary.

Another type of resonator which has superior performance to electronicresonators are acoustic resonators. A surface acoustic wave (SAW)resonator has a resonance frequency of 50-2,000 MHz. It also has a Qfactor of 4,000-15,000. A film bulk acoustic resonator (FBAR) has aresonance frequency of 300-10,000 MHz and a Q factor of 100 to 1000. Theuse of these types of resonators in the bandpass sigma-delta modulatoralso produces a digital output with high resolution at high frequencies.

FIG. 7 shows a bandpass sigma-delta modulator using an acousticresonator 2. The remaining parts of the circuit operate in a similarfashion to that of FIG. 3. Since the acoustic resonator 2 is a secondorder system, the bandpass sigma-delta modulator shown in FIG. 7 is ofsecond order and provides second order noise-shaping as shown in FIG. 8.This modulator using an acoustic resonator is capable of digitizing theanalog input signal, but with less resolution than a fourth ordermodulator.

FIG. 9 shows such a fourth order bandpass sigma-delta modulator usingacoustic resonators. This arrangement is similar to that shown in FIG.5, but uses acoustic resonators rather than micro-mechanical resonators.Since there are two acoustic resonators in the loop, the modulatorprovides a fourth order noise shaping as shown in FIG. 10 and istherefore of fourth order. This fourth order bandpass sigma-deltamodulator using acoustic resonators is able to provide the digitizationof the analog signal with a high resolution at high frequency.

Numerous additional modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A bandpass sigma-delta modulator comprising: a summing device forreceiving an input and first and second feedback signals and producingan output; a micro-mechanical resonator receiving the output of saidsumming circuit as an input and producing an output; a quantizerreceiving the output of said micro-mechanical resonator and producing adigital output which is the output of said modulator and; first andsecond D/A converters receiving said output of said quantizer as aninput and producing said first and second feedback signals.
 2. Thebandpass sigma-delta modulator according to claim 1, wherein gain stagesare provided between said first D/A converter and said summing device,between said second D/A converter and said summing device, and betweensaid micro-mechanical resonator and said quantizer.
 3. A bandpasssigma-delta modulator comprising: a first summing device receiving aninput, a first feedback signal and a second feedback signal andproducing an output; a first micro-mechanical resonator receiving saidoutput from said first summing device and producing an output; a secondsumming device receiving said output from said first micro-mechanicalresonator and receiving third and fourth feedback signals and producingan output; a second micro-mechanical resonator receiving said outputfrom said second summing circuit and producing an output; a quantizerreceiving said output from said second micro-mechanical resonator andproducing an output which is the output of said modulator; and first andsecond D/A converters, each receiving the output of said quantizer as aninput, said first D/A converter producing said first and third feedbacksignals, and said second D/A converter producing said second and fourthfeedback signals.
 4. The bandpass sigma-delta modulator according toclaim 3, further comprising gain stages between said firstmicro-mechanical resonator and said second summing device, between saidsecond micro-mechanical resonator and said quantizer, between said firstD/A converter and said first summing device, between said first D/Aconverter and said second summing device, between said second D/Aconverter and said first summing device and between said second D/Aconverter and said second summing device.
 5. A bandpass sigma-deltamodulator, comprising: a summing device configured to receive an inputsignal, and produce a first output signal; a micro-mechanical resonatorcoupled to the summing device and configured to provide a second outputsignal based, at least in part, on the first output signal; a quantizercoupled to the micro-mechanical resonator and configured to provide adigital output signal based, at least in part, on the second outputsignal; and first and second D/A converters coupled to the quantizer andconfigured to produce the first and second feedback signals based, atleast in part, on the digital output, wherein the first and secondfeedback signals affect a gain applied to the input signal.
 6. Thebandpass sigma-delta modulator of claim 5 , further comprising one ormore gain stages coupled to the first and second D/A converters andconfigured to apply the gain to the input signal based, at least inpart, on the first and second feedback signals.
 7. The bandpasssigma-delta modulator of claim 5 , further comprising: a first gainstage coupled between the summing device and the first D/A converter; asecond gain stage coupled between the summing device and the second D/Aconverter; and a third gain stage coupled between the micro-mechanicalresonator and the quantizer.
 8. A method, comprising: combining an inputsignal with a first analog feedback signal and a second analog feedbacksignal to apply a gain to the input signal and to provide a first outputsignal; receiving a second output signal from a micro-mechanicalresonator, wherein the second output signal is generated based, at leastin part, on the first output signal; quantizing the second output signalto provide a digital output signal based, at least in part, on thesecond output signal; and generating the first analog feedback signaland the second analog feedback signal based, at least in part, on thedigital output signal.
 9. The method of claim 8, further comprisingemploying one or more gain stages to apply the gain to the input signalbased, at least in part, on the first analog feedback signal and thesecond analog feedback signal.
 10. A cellular telephone, comprising: asoftware-defined radio configured to receive a digital signal; and abandpass sigma-delta modulator including: a summing device configured toreceive an input signal and produce a first output signal; amicro-mechanical resonator coupled to the summing device and configuredto provide a second output signal based, at least in part, on the firstoutput signal; a quantizer coupled to the micro-mechanical resonator andconfigured to provide the digital signal based, at least in part, on thesecond output signal; and first and second D/A converters coupled to thequantizer and configured to produce first and second feedback signalsbased, at least in part, on the digital signal, wherein the first andsecond feedback signals affect a gain applied to the input signal. 11.The cellular telephone of claim 10, wherein the bandpass sigma-deltamodulator further comprises one or more gain stages coupled to the firstand second D/A converters and configured to apply the gain to the inputsignal based, at least in part, on the first and second feedbacksignals.
 12. The cellular telephone of claim 10, wherein the bandpasssigma-delta modulator further comprises: a first gain stage coupledbetween the summing device and the first D/A converter; a second gainstage coupled between the summing device and the second D/A converter;and a third gain stage coupled between the micro-mechanical resonatorand the quantizer.